extremist works good too.
Fundamentalist atheist could work too but for intellectual integrity that requires the atheist in question to always be going “back to basics” and saying, “But the dictionary says [x] and because the dictionary says this, this means that’s all that matters”.

[the dictionary replacing Bible or other holy writ in that case].

But radical is actually not a bad word. It’s also Progressive and Far Reaching… it’s Thorough… has wide aims and lofty goals…

So really, if *I* was atheist (I’m not) and somebody called _me_ a radical atheist, I’d embrace the hell out of it and say, “Yeah, I am. Go on.”

The current marketed image of the atheist by atheists is the nebbish bookworm who is intellectual and smart and has something to say to the bloody horrible religious world and is an innocent in a world of brute savages.

But then you had the New Atheist movement which kinds wrecked that idea.